The present invention relates to a nonaqueous electrolyte battery using an iron compound as an active material for positive electrode, and more particularly it is intended to realize a stable nonaqueous electrolyte battery of high energy density utilizing iron which is abundant in resource and inexpensive, by forming iron compound powder as nano particles in an appropriate particle size range composed of primary particles of pore-free matter.
Recently, in the cordless and portable trend of electronic appliances such as audio and video apparatuses and personal computers, lithium secondary batteries of high energy density using nonaqueous electrolyte come to be employed widely. In these practical nonaqueous electrolyte lithium secondary batteries, a composite compound of much lithium and transition metal is used as the positive electrode active material. In particular, LiCoO2 is regarded as one of the important materials presenting a high operating voltage of 4 V class stably.
However, cobalt is a precious resource and the material cost becomes high, and an inexpensive positive electrode active material of high performance capable of replacing LiCoO2 has been demanded. From this point of view, the iron compound mainly composed of profuse and cheap iron has been one of important research objects. Regrettably, many problems are left unsolved, and it is not put in practical use at the present.
First, the iron compound has been questioned about possibility of basic electrochemical reactions such as operating voltage and discharge capacity. For example, as reported in Battery Discussion Papers 1995, pp. 23-24, LiFeO2 having zigzag layer structure was synthesized, and a nonaqueous electrolyte battery was fabricated by using it as the positive electrode, and its operation as a battery was confirmed. In this case, the discharge voltage was 2 V, and unlike the battery using LiCoO2 as the positive electrode, operation of 4 V class not observed.
B. Fuchs et al. reported that LiFeO2 of layer structure similar to that of LiCoO2, instead of zigzag layer structure, was synthesized (Solid State Ionics, 68, 1994, p. 279). Their report, however, did not include the action of the obtained LiFeO2 as positive electrode active material, that is, operation of insertion and desorption of lithium.
Japanese Laid-open Patent No. 8-78019 discloses a nonaqueous electrolyte battery using iron oxide containing lithium expressed in the formula LixFeOy (0<x 1.5, 1.8<y<2.2) as the positive electrode. Herein, the specific surface area of the iron oxide was measured by the BET method, and a preferable surface area was presented in a range of 0.5 to 20.5 m2/g. The battery obtained in the preferred condition operated in a 4 V region, and the discharge capacity per unit weight was 110 to 130 mAh/g. It is also disclosed that the preferable specific surface area range was obtained by selecting the particle size in a range of 0.4 to 20.5 m2/g. However, as mentioned in the patent publication, as the specific surface area increases, in the iron compound, the iron oxide containing lithium and electrolyte solution react, and the electrolyte solution is decomposed. Actually, the battery using the iron oxide having such large specific surface area presents a large discharge capacity per unit weight in the first cycle, but suddenly and substantially drops in the capacity after the second cycle, and the stability was very poor as the positive electrode active material for secondary battery.
On the other hand, it has been attempted to pulverize the active material further into a region of nano particles and increase the discharge capacity per unit weight of active material. For example, U.S. Pat. No. 5,569,561 discloses a technology of using oxides in nano particle form in a size of 1 to 250 nm, such as TiO2, Nb2O5, HfO2, MnO2, LiyNiO2, LiyCoO2, Liy(NiCo)O2, and LiMn2O4, as active material. In this United States patent, a battery using titanium oxide as the active material for negative electrode is disclosed, and the preferred particle size of this titanium oxide is specified to be in a range 1 to 300 nm. Although it is disclosed that the capacity is increased by using particles in a nano region, nothing specific is mentioned about the iron compound particles.
Separately, Japanese Laid-open Patent No. 9-82312 discloses a nonaqueous electrolyte battery using transition metal oxide or sulfide containing lithium composed of primary particles with particle size of 0.5 μm or less as the positive electrode active material. Nothing specific is mentioned about iron compound in this patent publication, but iron compound is included in the group of oxide and sulfide containing lithium, and a range of 5 nm to 200 nm is given as preferred particle size, and the range of preferred specific surface area is 100 m2/g or more. As disclosed also in this publication, the positive electrode active material powder has a peak of pore distribution at radius of 50 nm or less, and a section of a primary particle of porous matter is illustrated.
As explained in Japanese Laid-open Patent No. 8-78019, although it is affirmatively known that the initial capacity is heightened by pulverizing the active material and increasing the specific surface area, if using the iron oxide containing lithium as positive electrode active material, the electrolyte solution is decomposed when the specific surface area exceeds 100 m2/g, thereby producing a contradictory result of disturbance of stability of characteristic. That is, by nano pulverization, particles increase in the region inducing such problems.
These prior arts may be summed up as follows. That is, by synthesizing a layer structure similar to LiCoO2, and selecting a region of large specific surface area in a fine nano particle region, it was once considered possible to apply the iron compound in the active material for nonaqueous electrolyte battery of 4 V class. Actually, however, the specific surface area increases by nano pulverization, and there is a risk of inducing a serious problem for iron compound such as decomposition of nonaqueous electrolyte. From the viewpoint of realizing a stable secondary battery of high capacity by applying the iron compound as the active material, it seemed necessary to develop a new particle control technology that cannot be controlled by the nano pulverization of particles alone. In other words, it is a new and vital issue for using iron compound as nonaqueous electrolyte secondary battery to solve simultaneously two contradictory problems, that is, to heighten the capacity by using nano particles, and to avoid decomposition of electrolyte solution due to pulverization of particles.